Many types of sorbent particles are used to remove solutes from gaseous or liquid solvents, either to isolate and concentrate resource materials or to remove hazardous wastes or pollutants. A sorbent is generally defined as a material, compound, or system that can provide a sorption function, such as adsorption, absorption, or desorption. A solvent is generally defined as that part of a solution that is present in the largest amount, or the compound that is normally liquid in the pure state. A solute is generally defined as the substance dissolved in a solvent. When a compound is dissolved in a solvent, its components usually dissociate into their ionic forms. The ion with a net positive charge is referred to as the cation, whereas the ion with a net negative charge is referred to as the anion. For example, a metal-containing compound such as silver nitrate would dissociate in solution into silver cations and nitrate anions. Other metal-containing compounds generally behave in the same way in solution. Of particular interest is the recovery of metallic ion species, especially metallic ion species with an atomic weight of about 50 or greater, from various liquids. These metals encompass a large number of elements, including but not limited to, metals such as copper, uranium, strontium, cadmium, silver, mercury, iron, and lead. In a liquid containing at least one type of metallic ion specie, the solute would be the metallic ion specie, whereas the liquid would be the solvent. The material used to remove the solute from the solvent is the sorbent. In such a case the solute would also be referred to as the sorbate. A sorbate is generally defined as the substance sorbed by a sorbent.
Conventional fluidized-bed contactors usually operate as a columnar chamber through which there is an upflow or fluid that suspends the sorbent particles in the active portion of the contactor. The fluidized sorbent particles tend to migrate throughout the expanded bed with constant movement while the fluid progresses up through these sorbent particles, essentially as a front as in plug flow. Plug flow, also known as slug or piston flow, is generally defined as a fluid flow in which the fluid flows as a front with no mixing or diffusion along the flow path.
In conventional fluidized-beds essentially all of the sorbent particles will be contacted with high concentrations of tile solute over a relatively short period of time, therefore the effluent concentration of the solute will tend to breakthrough or increase after a short time of contact. Breakthrough is generally defined as the first appearance in the effluent of unsorbed solute of the type which is being sorbed by the sorbent. This usually indicates that the bed needs to be regenerated, causing the entire contactor to be taken off-line.
A fixed-bed of sorbent particles represents a somewhat more efficient contractor with later breakthrough. However, it has a major disadvantage in that after the sorbent particles are loaded with the solute, the entire contactor must be taken off-line while the sorbent is regenerated. On the other hand, the sorbent particles in fluidized-bed contactors are relatively easy to add or remove on a continuous or semi-continuous basis so that such a contactor could be operated continuously with the sorbent particles regenerated in a separate unit and recycled back to the contactor.
There is a need for a fluidized-bed contactor system in which fresh sorbent particles tend to migrate to the portion of the contactor with the highest concentration of solute, while solute-loaded sorbent particles are segregated in another area within the fluidized-bed where they can be easily removed.